This invention relates to an apparatus for recording images on an electrographic medium and in particular to an electrophotographic recording system that employs two corona elements that deposit charge on a photoconductive surface of the electrophotographic medium. The polarity of the charge deposited by the second corona element is opposite to that of the charge deposited by the first corona element and the second corona element is automatically controlled to maintain a conductive layer of the recording medium at a constant potential.
In one type of electrophotographic recording process, the recording medium is a film having a polyester base, a transparent conductive layer that overlies the polyester base and a photoconductive layer that overlies the transparent conductive layer. To record on the film, a uniform electrostatic charge is first applied, by means of a charging corona element, to the surface of the photoconductive layer of the film. The uniform charge is then selectively dissipated by exposing the surface to a light image of the pattern to be recorded. The resulting pattern of charges is an electrostatic latent image on the photoconductive surface which can then be rendered visible for example, in an attraction toning process, by applying thereto electrostatically charged developer particles which are held to the surface of the photoconductive layer by means of the electrostatic force developed between the developer particles and the charge on the photoconductive surface. A permanent visible image can be obtained, for example, by using developer particles which can be heat fused to the photoconductive layer, and then subjecting the visible image to a heat fusing step.
During the recording process it is desirable to maintain the transparent conductive layer of the film at a constant potential. This was initially accomplished in prior art recording systems by providing an electrical contact to the transparent conductive layer and then connecting the electrical contact to ground in the recording system. Because the transparent conductive layer is very thin, typically on the order of 0.01 micrometers, and because the transparent conductive layer is sandwiched between the polyester base and the photoconductive layer, it is very difficult to provide a good, reliable electrical connection to the transparent conductive layer. It would, therefore, be desirable to provide a recording system in which the transparent conductive layer is maintained at ground potential without making electrical contact to that layer of the film. U.S. Pat. No. 3,779,749--Sato eliminates the need to provide an electrical contact to the transparent conductive layer of the film. In the Sato patent, a second corona element, located after the development stage of the recording apparatus, deposits a charge of the opposite polarity of the charge deposited by the charging corona element onto the photoconductive surface of the film. The voltage applied to each corona element is manually adjusted so that the magnitude of both corona currents are equal. Thus, the amount of positive charge deposited on the photoconductive surface by one corona element is, in theory, balanced out by the amount of negative charge deposited on the photoconductive surface by the other corona element. Since the voltage of the conductive layer is determined by the magnitude of the net charge on the photoconductive layer, if the net charge is zero, then the conductive layer will be at ground potential.
One disadvantage of the recording system described in the Sato patent is that even though a constant voltage is applied to a corona element, the magnitude of the corona current and, therefore, the charge deposited on the photoconductive surface will undergo considerable variation depending upon the environmental conditions. Thus, temperature, relative humidity, chemical composition of the environment, and particularly barometric pressure will all affect the corona current level. It is also well known that environmental conditions do not affect both positive and negative corona elements to the same degree. Thus, as the environmental conditions change from those that existed when the voltage applied to the two corona elements were intially adjusted, the two corona elements will tend to put a net charge of one polarity or the other on the photoconductive surface of the film.
Another disadvantage of the recording system described in the Sato patent is that the system does not continually compensate for other sources of charge variation inherent in the recording process. For example, since the recording process includes an exposure step wherein charge deposited on the electrophotographic surface of the medium is selectively dissipated, different recorded images will generally cause different amounts of charge to remain on the photoconductive surface of the medium. Also, since the toner particles employed in the development step are themselves charged particles, they will also have an affect on the net charge on the photoconductive layer of the medium. Given the above sources of charge variation it can be appreciated that even though two manually adjustable corona elements are provided, a significant variation in the net charge on the surface of the photoconductive layer can occur which will result in a variation of the potential of the conductive layer of the recording medium.
It is, therefore, the primary object of this invention to provide an improved apparatus for recording on an electrographic medium that reduces the variation of the potential of a conductive layer of the recording medium.
It is another object of this invention to provide an improved apparatus for recording on an electrographic medium that maintains a conductive layer of the medium at a constant potential despite variations in the net charge on the surface of the recording medium that result from recording different images on the medium or from applying charged toner particles to the medium.
It is another object of this invention to provide an apparatus for recording on electrophotograhic film that includes a charging corona element and a discharging corona element wherein the current level of the discharge corona element varies as a function of the magnitude of the voltage on a conductive layer of the electrophotographic film.
And yet another object of this invention is to provide a system for recording on electrophotographic film that includes a charging corona element and a discharging corona element wherein the discharging corona element is automatically controlled in order to maintain the conductive layer of the film at a constant potential even though environmental conditions affect the voltage versus current relationship of the corona elements.